Cadmium iodide

Cadmium iodide is an inorganic compound with the formula CdI2. It is a white hygroscopic solid. It also can be obtained as a mono- and tetrahydrate. It has few applications. It is notable for its crystal structure, which is typical for compounds of the form MX2 with strong polarization effects.

Preparation
Cadmium iodide is prepared by the addition of cadmium metal, or its oxide, hydroxide or carbonate to hydroiodic acid. Also, the compound can be made by heating cadmium with iodine.

Applications
Historically, cadmium iodide was used as a catalyst for the Henkel process, a high-temperature isomerisation of dipotassium phthalate to yield the terephthalate. The salt was then treated with acetic acid to yield potassium acetate and commercially valuable terephthalic acid.

While uneconomical compared to the production of terephthalic acid from p-xylene, the Henkel method has been proposed as a potential route to produce terephthalic acid from furfural. As existing Bio-PET is still reliant on petroleum as a source of p-xylene, the Henkel process could theoretically offer a completely bioplastic route to polyethylene terephthalate.

Crystal structure


In cadmium iodide the iodide anions form a hexagonal closely packed arrangement while the cadmium cations fill all of the octahedral sites in alternate layers. The resultant structure consists of a layered lattice. This same basic structure is found in many other salts and minerals. Cadmium iodide is mostly ionically bonded but with partial covalent character.

Cadmium iodide's crystal structure is the prototype on which the crystal structures of many other compounds can be considered to be based. Compounds with any of the following characteristics tend to adopt the CdI2 structure:


 * Iodides of moderately polarising cations; bromides and chlorides of strongly polarising cations
 * Hydroxides of dications, i.e. compounds with the general formula M(OH)2
 * Sulfides, selenides and tellurides (chalcogenides) of tetracations, i.e. compounds with the general formula MX2, where X = S, Se, Te